Fig. 3.

(A) Cross-correlation as a function of frequency of the ECoG and the speech envelope; same sentence as in Fig. 1C. Data have been separately normalized on the different frequency bands. Notice that maximal correlation values are within the theta band (2–8 Hz). Frequency (Hz): Frequency of the ECoG in Hz. ΔT[s]: time differential in seconds between the ECoG and the audio trace of overt speech; ECoG response always anticipates sound. (B) Plot showing the dependency of the amplitude of cross-correlation (Correlation %) of the ECoG and the speech envelope (left y axis) on the correct linear order of the original signal. When we randomly shuffled subintervals of decreasing length obtained by cutting the audio and ECoG trace of the same sentence represented in Fig. 1C and A, we obtained an exponential decay of the correlation amplitude that halved for an interval duration of ∼0.09 s. Red circle: Correlation value obtained of each shuffling; the black line fits the mean values of the correlation amplitude obtained in all independent shuffling for each subinterval length. Subinterval n.: Number of possible subintervals for each interval duration (right y axis) is shown by blue triangles. Duration (s): Length of subintervals in seconds. (C) Amplitude of the cross-correlation above background maximal value (20%) of the ECoG and the speech envelope as a function of the linear distance (cm) of the recording electrodes from the electrodes overlaying the area in which electrical stimulation induced speech arrest (0). Temporal electrodes, indicated by green “+,” were all referred to the average distance of 5 cm. Electrodes located over the anatomical Broca’s area are indicated by red “x”; frontal electrodes outside anatomical Broca’s area are indicated by cyan circles. All data (90.504 measures) obtained in all patients at all frequencies of audio and ECoG bands are represented. No significant differences in the distribution of the correlation amplitudes are visible between electrodes located over the speech arrest area and the electrode outside it. Please notice that electrodes located over the superior parietal gyrus are not represented here. (D) Spatial distribution of all of the electrodes showing a suprathreshold value (20%) for the correlation between the audio envelope and ECoG simultaneous to the correlation peak of one electrode located either over an area of the frontal cortex whose stimulation did not interfered with language [E2 (Premotor)] or over another area of the cortex whose stimulation induced speech arrest [E13 (Broca)]. Each square represents an electrode; colors (red, maximal; blue, minimal) are proportional to the frequency of occurrence of suprathreshold correlation values and are indicated by the number in the middle of the square. Bigger rectangles (Frontal) represent electrodes located over the dominant frontal lobe, and smaller rectangles (Temp.) represent electrodes located over the dominant temporal lobe. A color scale is provided with the indication of percentiles corresponding to each color. The probability distribution of a suprathreshold correlation changes significantly according to the position of the reference electrode considered. The number of electrodes in the dominant temporal lobe that reach suprathreshold correlation values simultaneously with the electrode located in the area of speech arrest is significantly higher than when the electrode was located on a prefrontal area, where speech arrest could not be evoked.